1. Field of the Invention
The present invention relates to HVAC systems and more particularly to an HVAC system using a multi-source heat pump.
2. Description of the Prior Art
Heretofore, most buildings utilized natural gas or oil fired heaters for space heating and electrically operated air-conditioning units for space cooling. Since the energy crisis, the cost of all forms of energy has risen sharply with the cost of oil increasing most drastically and the cost of electricity rising the least. The cost of natural gas which for years was the cheapest form of energy, may go considerably higher should it be deregulated by the Government and in many areas additional use of natural gas has been prohibited. Thus, electrical energy is becoming the most desirable and economical form of energy even for space heating purposes. Attempts have been made to use electric furnaces having electric resistance heaters; however, experience has indicated that these types of devices are inefficient and much too expensive for acceptable use.
Since most modern buildings require both heating and air-conditioning, the trend has been to use combined HVAC systems to provide the most economical heating, ventilation, and air-conditioning for the entire building. Various types of HVAC systems have been used such as standard four-pipe central heating and cooling systems and variable air volume cooling systems with baseboard radiation for heating. Multi-zone systems having both hot and cold decks and closed loop water source heat pumps have been used in an attempt to use excess heat from one zone for heating a cooler zone; however, these systems all suffered from various deficiencies ranging from expensive initial installation to poor economy of operation.
Since there was a requirement for both heating and cooling, it appeared as if reversible refrigeration systems, known as heat pumps, might provide the most economical solution especially since these devices use electrical energy and have very high coefficients of performance not realized by other types of heating devices. Most prior art residential heat pumps were of the air-to-air type wherein an outdoor heat exchange coil was used to either extract heat from the ambient air or to discharge heat to the air depending upon the particular mode of operation, and an indoor heat exchange coil was used to either heat or cool the room air. One difficulty experienced with this type of device was that when the outdoor coil was used to extract heat from the ambient air during cold ambient temperatures, the low operating temperatures caused frost to build up on the outdoor coil, necessitating the use of an energy-consuming defrost cycle which substantially limited the efficiency and usefulness of air-to-air heat pumps. When a water-to-air heat pump was used, the problems associated with frost build-up were avoided because the heat could be extracted from the water without having to operate at the low temperatures required for an air-to-air heat pump. However, a source of constant temperature water was required and it was usually necessary to provide a rooftop evaporative cooler for discharge of heat during the cooling mode of operation.
In an attempt to make more efficient use of heat pumps, systems were designed wherein a mechanical refrigeration type air conditioning system was provided for an interior zone with the condenser of said system being in heat exchange relationship with a closed water loop that functions as a water source for peripheral zone heat pumps. Such devices reduced energy consumption to a degree but still relied upon mechanical refrigeration and the heat discharged by the condenser to provide heat to the closed water loop used by the peripheral zone heat pumps.
None of the prior art HVAC systems took into consideration the fact that a substantial portion of the building heat was lost by exhausting warm air at various places throughout the building.